(a) Field of the Invention
This invention relates to the treatment of particulate matter in regions affected by electric arc discharges, and in particular to the interaction of low temperature plasmas (i.e., below 100,000 K.) formed in such discharges with dense populations of particles entrained therein.
Plasma may be defined, in current terminology, as an assembly of electrically charged particles exhibiting collective behaviour. Such assemblies may be provided, for example, by a body of hot ionized gas in which the numbers of positive charges (ions) and negative charges (electrons) are usually, but not necessarily, substantially equal, resulting in overall electrical neutrality or `quasi-neutrality`. The expression `low temperature plasma` is here taken to refer arbitrarily to plasmas having an ion temperature below 100,000 K. The arc discharges employed herein are to be distinguished from glow discharges which occur at considerably reduced pressures. In the present invention the discharge will usually be conducted in a medium at or above substantially atmospheric pressure, although considerable local variations in pressure may be present, and indeed form a preferred feature of the invention.
(b) Description of the Prior Art
Low temperature plasmas have received considerable attention in the last decade as a possible alternative route to many industrial processes, such as the manufacture of steel and ferro-alloys, various syntheses and manufacture of pozzolanic materials, and hydraulic cements.
All the suggested methods, whatever their design or arrangement, have one thing in common: they all rely on the conversion of electric current into high temperature effluents of one kind or another which, by virtue of their high temperature and enthalpy, interact with the feedstocks, bringing about a higher rate of reaction than would take place at a low temperature. In this sense, all these methods may be locked upon as predominantly thermal and the reactions occurring take place between the high temperature gaseous (or partially ionised) phase and the solids. Increasing the temperature of the feedstocks to a level high enough to make them dissociate, although possible, is far too expensive in electrical energy to make such a method industrially attractive. On the other hand, most conventional methods of producing low temperature plasmas, for example, plasma torches (`plasmatrons`) based on radially constricted arc discharges or RF torches, generate far too small volumes of plasma at far too high temperatures to make them industrially applicable.
Various attempts have been made in the past to introduce particulate materials into low temperature plasmas. For example, introduction of particles into the arc chamber of a plasma torch has been tried, but this causes serious problems. Among these is contamination of cathode, on the low work function of which the efficiency of electron emission depends, and frequent unacceptable erosion of the constricting channel of the torch, as well as accretions. Introduction of particles into the constricting channel itself, on the other hand, disturbs the plasma and seriously limits the quantity of particles that can be introduced. This mode of particle introduction is, however, practised in so-called `plasma spraying`, where the introduction of only relatively small amounts of particles is required.
There are also more general difficulties in the treatment of particles in plasma, of which the most important are:
1. Difficulties with the introduction and retention of particles in the plasma of the arc discharges. These are chiefly due to high viscosity gradients between the plasma and the surrounding gas and to thermophoretic phenomena which tend to reject such particles from the plasma zone.
2. Difficulties in maintaining arc discharges in the presence of particles which, when present in larger quantities, tend to extinguish such discharges chiefly by the development of various electron-scavenging mechanisms which capture the current-carrying electrons in the plasma.
3. Difficulties in providing the same treatment for all the particles entering the reaction zone, irrespectively of their size. Thus when larger particles may not yet be fully treated, smaller ones may already have partially or wholly evaporated, bringing about electron-scavenging and resulting in instability and collapse of the discharge.
4. Further difficulties occur with the choice of materials for the electrodes and the refractories of such devices. If, as is commonly done, direct current is employed, the cathode is frequently in the form of a non-consumable thoriated tungsten rod in a plasma torch working in the transferred mode. However, in such circumstances the anode to which the arc is transferred dissipates large quantities of expensively derived energy and consequently must be intensively cooled, which represents considerable energy losses. Similarly, the refractories containing the reaction zone are kept small in order to engage as many particles in the plasma as possible. As a result of this, frequent failures of refractories occur and, furthermore, the surfaces of many refractories, particularly when contaminated with fumed feedstocks, become electrically conducting and give rise to short circuits.
5. As such devices require particularly critical current and voltage control, when scaled up to even a few megawatts of power, their direct current supplies become very cumbersome and expensive, while the increased power in a plasma torch makes all the above listed difficulties more acute.
The above difficulties were well recognised by those skilled in the art and, as a result, the industrial applications of low temperature plasma technology followed, broadly speaking, two different routes.
The first route, in which the volume of the arc discharge was not used for entrainment of feedstocks, chiefly utilised the point of impingement of the arc at the anode and behaved in this respect very much like electric arc furnaces. Their main advantages are claimed to be the use of a non-consumable electrode (namely, a plasma torch) and the fact that the high kinetic energy of the plasma effluent causes stirring of the melt thereby distributing its temperature. A variety of such devices based on one or more plasma torches, or plasma torches operating in combination with orthodox electric arcs, are described in the literature and operate usually at a pilot plant level. In some of these devices, solid particles are introduced during the arc operation but their interaction with the latter is minimal.
The second route aims at the treatment of particles in the whole volume of the plasma produced. For this purpose it was required to expand the plasma, and this involves increasing its original volume with simultaneous reduction of its temperature and viscosity gradients. Two entirely different methods have been used for this purpose. The first of these was based on the fact that when an electric arc discharge is placed symmetrically in the centre of a hollow rotating cylinder it begins to expand radially outwards due to viscous drag forces, until at a certain angular speed it fills the whole of the cylinder. This principle was originally described by W. Weizel et al., in `Theorie Elektrischer Lichtbogen und Funken`, Barth, (Leipzig, 1949). A number of devices based on this principle were constructed, but there are serious limitations to this method. These are chiefly due to the rapid rotation of a large cylindrical body which such a furnace requires, and the fact that only when the plasma is fully expanded does it become stable. However, when this takes place, the plasma is in contact with the inner rotating refractory walls and tends to destroy the latter. Chiefly for these reasons, this technique found only limited applications and did not prove itself capable of treating large quantities of plasma-entrained particles.
The second method of expanding low temperature plasma arc discharge was discovered by the present inventor in 1971 and disclosed in British Patent Nos. 1,390,351-3. In this method a plasma torch acting as a cathode was made to orbit in a circular path and at a small angle with the vertical, projecting the arc to a downstream annular anode. In this way a truncated conical region was defined by the orbiting arc discharge. This method, which was in turn derived from previous work of the same inventor on planar expansion of plasma jets as described in British Patent No. 1,201,911, aimed at the formation of a large volume of plasma in which solid particles could be treated. This method, which became known as Expanded Precessive Plasma (E.P.P.'), showed certain advantages over the previous method when practised intermittently and on a small scale, of approximately 1 to 2 MW, and served well as a laboratory plasma furnace for studying many reactions. However, its main disadvantage proved to be the limited orbiting speed of the plasma torch and the need for frequent replacement of the consumable anode. During its eight years of development, various methods of oribiting the torch were tried but in view of the large out-of-balance inertia forces which inevitably develop in an inclined plasma torch, the maximum angular speed reached was 2000 rpm, while a safe operational speed was considered to be 1500 rpm. These low speeds were chiefly responsible for the very limited expansion of the primary plasma jet, as was confirmed by photography. As a result of this there was only a limited and often sporadic interception of the falling particles by the plasma jet, and the method was limited to the injection of relatively small amounts and not uniformly dispersed feedstocks.
The lack of suitable control to provide for uniform exposure of the falling solids to the plasma was the main limitation of this technique. As mentioned above, mere increase in the power of the primary plasma jet reduces interception and increases the rate of rejection of the particles. These phenomena were further confirmed in more recent work of the same inventor, described in British patent applications Nos. 45839/76 and 28881/77, in which the EPP technique was used for making pozzolanic materials from colliery spoils. In that case, the lack of uniformity in the products further confirmed this limitation. However, when used in melting, that is, transferring the primary plasma jet to an electrically conducting melt, the EPP method behaved similarly to the methods of the first group of devices, as mentioned above.
It is also thought that scaling-up of EPP installations to full industrial requirements may prove difficult. While large plasma torches are available, making them orbit at high angular velocities is more difficult. Such torches are also likely to cause serious ablations when their arcs are transferred to the anodes. Finally, it should also be said that while EPP installations require complex controls in order to maintain their arcs, there are no means for adequately controlling the processes responsible for the entrainment and treatment of the particles themselves. Consequently, the efficiency of these processes is not as high as it might have been if a larger portion of the energy of the plasma effluents could have been utilised.